Non-woven fabrics have achieved very large use where they are suitable because they can be made at high speed of low cost raw material, which reduces the final cost of the fabric far below woven or knitted fabrics. Two general types of non-woven fabrics are those where the fibers are felted and those in which weft yarns are laid across warp yarns or are looped around a set of warp yarns. Sometimes this is effected by spiraling the weft yarn around warp yarns. It is with the second general type of non-woven fabric, i.e. the type with laid in weft yarns, that the present invention deals.
In general non-woven fabrics of layers of warp and weft yarns are subjected to treatment with coating compositions, and particularly adhesive compositions, which glue the weft yarns and warp yarns together, thus locking them. Where the dimensional stability of locking is not essential but where it is desired to alter the surface of the yarns by coatings, this has also been effected. In general the two types of fabrics just described are really the same, as whether there is actually adhesive locking or mere coating of the fibers depends on the chemical and physical nature of the coating compositions with which the fabric is treated.
Spiraling weft thread around a layer of warp threads may be effected in various manners, for example by a spiral wrapping, a typical example of which is described in the U.S. Pat. No. 3,041,230 to Diehl, or by feeding spiral weft loops along two outer warp threads acting as selvage threads by having the selvage threads pass through a rotating corkscrew-shaped scroll which carries loops of weft thread and deposits them across these warp threads. This modification of the spiraling of weft threads around warp threads is described, for example, in the U.S. Pat. No. 3,422,511 to Seguin.
Generally where weft thread is spiraled around warp threads, at this point the product may be considered as a multi-layer laminate with warp threads between layers of weft threads. The various modifications of the spiraled weft thread type of fabric are useful products and can be practically produced. As with many products of practical utility, there are certain characteristics which are not ideal. First, friction in the scrolls can damage certain yarns, such as glass yarn. Secondly, considering only non-woven fabrics which are held together by treatment with adhesive compositions, it is desirable to have the weft threads and the warp threads held in a definite predetermined position as they are treated with the adhesive, followed by nip rolls and heating or other means for setting the adhesive or binder. When the weft thread is spiraled around some or all of the warp threads, the resulting non-woven fabric must move to adhesive treatment operations, and when so moving the unlocked warp threads and the spiral weft threads can move over each other. In other words, the warp threads may move sideways of the fabric, resulting in some warps crossing each other, and the weft spirals can move varying the closeness with which the spirals are laid. For many fabrics slight moving before locking by adhesives is not serious, and so this characteristic may be unobjectionable in such fabrics. However, for certain fabrics where it is desired to maintain exact alignment of warp threads and weft threads, this constitutes a drawback.
Laying of parallel yarns as a broad concept is not unknown in the non-woven fabric art. For example, a mechanism is described in the U.S. Pat. No. 3,390,439 to Kalwaites. This leads yarn from a supply to hooks on a pair of diverging hook carrying chains, and at the point of maximum divergence the parallel line loops drop off the hooks onto a conveyor belt. With certain types of strong yarns or yarns which are not excessively lively, that is to say, which do not tend to twist or snarl, quite rapid operation is possible. The patent is not particularly concerned with non-woven fabrics which also have warp threads; it is primarily concerned with cross laying the yarns for yarn reinforced papers and the like, where they are applied, usually with binders, to the paper. If, however, the parallel yarns on the conveyor belt are laid across warp yarns to produce a warp and weft non-woven fabric, the friction of the warp yarns can cause some movement of the parallel weft yarns before adhesive locking, and so while there is no problem of moving or crossing warp yarns, there is a problem with the weft yarns on the conveyor when they are transferred to the moving warp yarns before the resulting fabric is locked. Here again useful fabrics or materials represent some degree of compromise, which with fabrics for certain purposes is not desirable.